The present invention relates to a control apparatus for an internal combustion engine which performs multivariable control to approximate a dynamic model of an internal combustion engine taken as a target of control and thereby causes the behavior thereof to approach a target value. More particularly, this invention relates to a control apparatus structure which optimally suppresses effects exerted upon control results by modeling error arising from load fluctuations or the like of the internal combustion engine approximated as the dynamic model.
Known control apparatuses of this type include, for example, an apparatus disclosed in Japanese Patent Application Laid-open No. 64-8336 (U.S. Pat. No. 4,785,780), an apparatus disclosed in Japanese Patent Application Laid-open No. 4-5452, and an apparatus disclosed in Japanese Patent Application Laid-open No. 4-279749 (U. S. Pat. No. 5,184,588). Each of these control apparatuses considers the internal combustion engine as a dynamic system with consideration to the internal state of the engine, determining input variables of the engine while estimating the dynamic behavior of the engine by means of state variables which prescribe the internal state thereof. I.e., a method of state variable control based on what is known as modern or advanced control theory is employed in order to control the speed of the internal combustion engine when idling i.e., the idle speed.
Normally, as a means for estimating the internal state of an internal combustion engine which is the controlled object in this type of state variable control based on modern control theory, a state monitor termed an observer is employed. The observer's role to periodically estimate state variable quantities of the internal combustion engine from operating quantities (control input information) of the engine and control quantities (control output information) of the engine. However the apparatuses described in these publications are made to output specific control quantities and operating quantities such as the speed of the internal combustion engine and the operating quantity of idle air as state variable quantities representing the internal state of a dynamic model of the internal combustion engine, thereby obviating the construction of this observer and even alleviating complication when modeling the controlled object. The state variable quantities which are output in this manner undergo, for example, integral compensation according to the accumulation value of the difference from the target value of the idle speed detected as the above-mentioned control quantity. Furthermore, as an operating quantity capable of converging the state feedback system at high speed on the basis of the predetermined optimal feedback gain of the relevant model, it is given to an actuator which acts upon, for example, the above-mentioned idle air.
By providing the above-mentioned control apparatus of the prior art with means for outputting specific control quantities and operating quantities as state variable quantities representing the internal state of the dynamic model of the internal combustion engine in this manner, reliable error-free accuracy and prompt control are made possible for the relevant state variable quantities while having a comparatively simple control device structure which obviates the construction of the above-mentioned observer.
It is to be noted that the above-mentioned state variable quantities themselves are output as values tracking the internal fluctuations of the internal combustion engine taken as the controlled object, and this makes for a control method in which this state variable control performed on the basis of modern control theory is resistant to modeling error.
With regard to the above-mentioned optimal feedback gain, however, because this is normally predetermined as a coefficient which is specific to the internal combustion engine approximated as the dynamic model, the modelling errors cannot be ignored in the event that such modeling error occurs. For this reason, if large fluctuations occur in the internal combustion engine taken as the controlled object, the reliability of the feedback gain itself becomes doubtful, and desirable state feedback is not necessarily maintained as the control apparatus.